15.8
Group Design
The most basic experimental design involves two groups: the experimental group and the control group. The two groups are designed to be the same excep…
When deciding how to structure a study that involves groups of people, researchers typically choose between two experimental designs: between-groups and repeated-measures or within-subjects.
Perhaps a researcher wants to compare how much readers empathize with the protagonist in a story—the dependent variable—when they read text that contains more emotional language versus more descriptive wording—the two levels of the independent variable.
She may decide to use a between-groups design—randomly assigning different individuals to each condition: the emotional language and descriptive language groups. As a result, each participant will only take part in one condition.
With random assignment, any potential confounds, such as how much people normally empathize with characters while reading, should be more equally distributed across both groups.
Alternatively, the researcher may decide to carry out the same study using a repeated-measures design—assigning the same participants to every level of the experimental conditions. Here, after taking part in one condition, each participant will also complete the second condition.
In this repeated-measures, or within-subjects design, potential confounds and the amount of error due to natural variance are automatically controlled for because the same participants are in each group.
For this reason, there is also the risk of order effects—participants performing differently in each condition because of the order they were presented.
To avoid this issue affecting the results, counterbalancing—an approach that ensures all possible orders of the conditions occur—is used. For instance, half the participants could be asked to read the emotional text first, while the other half reads the descriptive text first.
In the end, both designs have their pros and cons when it comes to collecting and analyzing data, so plan accordingly!
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Q1: What is the difference between between-groups and repeated-measures designs?
Between-groups design randomly assigns different participants to each experimental condition, so each person experiences only one level of the independent variable. Repeated-measures design assigns the same participants to all conditions, meaning each person completes every level. Repeated-measures automatically controls for individual differences but risks order effects, which counterbalancing can mitigate.
Q2: Why is random assignment critical in experimental research?
Random assignment ensures all participants have an equal chance of being assigned to experimental or control groups, making systematic differences between groups unlikely. This allows researchers to assume any observed differences result from the independent variable manipulation rather than preexisting characteristics. With sufficiently large samples, random assignment distributes potential confounds equally across groups.
Q3: How do independent and dependent variables relate in an experiment?
The independent variable is manipulated by the experimenter and represents the treatment or condition being tested. The dependent variable is what the researcher measures to assess the independent variable's effect. The dependent variable depends on the independent variable, answering the question: what effect does the independent variable have on the dependent variable?
Q4: What is counterbalancing and why is it used in repeated-measures designs?
Counterbalancing ensures all possible orders of experimental conditions occur across participants, preventing order effects from biasing results. In a repeated-measures study, participants might perform differently based on condition sequence rather than the independent variable itself. By having half the participants experience conditions in one order and half in the reverse order, counterbalancing controls for this confound.
Q5: What role does operationalization play in experimental design?
Operationalization is a precise description of how variables will be measured, ensuring clarity and reproducibility. For example, defining violent behavior as only physical acts like kicking or punching versus including verbal aggression. Clear operational definitions allow other researchers to understand exactly what was measured and replicate the study accurately.
Q6: How does random sampling help ensure representative research findings?
Random sampling gives every population member an equal chance of selection, creating a sample that reflects the larger population's characteristics. When samples are large enough and representative, researchers can generalize findings to the broader population without bias. This approach ensures percentages of demographic characteristics in the sample closely match those in the target population.
Q7: What are confounding variables and how does experimental design control them?
Confounding variables are unknown factors that could influence results independently of the independent variable. Random assignment distributes potential confounds equally across experimental and control groups, preventing them from systematically affecting one group more than another. In repeated-measures designs, using the same participants automatically controls for individual difference confounds.